1. Field of the Invention
The invention relates to a steering system.
2. Description of Related Art
Vehicular systems include various systems such as a steering system, a brake system, a back guide system, and a vehicular stability control system which perform control using an absolute steering angle. For example, in a steering system described in European Patent No. 2050658, the absolute steering angle is detected using an evaluation unit configured as an ASIC including a sensor that can detect a relative steering angle and a counter that increases and decreases a count value in accordance with a relative steering angle signal obtained from the sensor even while an ignition switch is off (during an IG off state). That is, to what period the steering angle corresponds is determined based on the count value, and the absolute steering angle is evaluated using the relative steering angle detected by the sensor.
If an abnormality such as a mismatch between the relative steering angle signal and an increase or a decrease in count value occurs in the evaluation unit, an incorrect absolute steering angle may be detected. Detection of the incorrect absolute steering angle causes a controller for the steering system to perform control based on the incorrect absolute steering angle. Thus, the system in European Patent No. 2050658 detects an abnormality in absolute steering angle as follows. That is, two such counters as described above are provided to make a count function redundant so that an abnormality detecting unit can compare two count values to determine whether the detected absolute steering angle is normal. Upon determining the absolute steering angle to be abnormal, the controller for the steering system stops the control based on the absolute steering angle.
The steering system in European Patent No. 2050658 certainly enables an abnormality in absolute steering angle to be detected. However, the system needs processing time to compare the two count values. The time needed to determine an abnormality in absolute steering angle is also increased by an amount equal to the processing time.